AN2644 datasheet(33/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

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t6 → t7. This is the deadtime during which both Q1 and Q2 are OFF. At t=t6

IR = I(Lp) is (in absolute value) greater than zero and provides the energy to let the

node HB swing from 0 to Vin, so that the body diode of Q1, DQ1, is injected. This

allows IR to flow back to the input source. The voltage across Lp reverses to a

Vout, D1 starts conducting while D2 is reverse biased with a negative voltage

approximately equal to 2·Vout (plus the contribution from LL2, here not shown).

This phase ends when Q1 is switched on at t=t7.

t7 → t8. Q1 is ON and Q2 is OFF. At t=t7 IR is diverted from DQ1 to the RDS(on) of

Q1, so that no significant energy is lost during the turn-on transient. Note that now

the operating point of Q1 is in the third quadrant, current is flowing from the source

to drain. This phase, along with the preceding one is the "external energy

recirculation phase": current is negative (coming out of the input terminal) despite

that the impressed voltage is positive so that the input energy is negative, i.e. it is

returned to the input source. D1 is still conducting and the voltage across Lp is a

Vout, so that Lp is not participating in resonance anymore and Cr is resonating with

Ls only. IR is again a portion of a sinusoid having a frequency f = fR1. This phase

ends when IR=0 at t=t8 and another switching cycle begins.

Remarks

Also in this "below-resonance" DCM submode the multiresonant nature of the LLC

converter shows up. From t1 to t2 and from t5 to t6 the secondary rectifiers are both

open and the second resonance frequency fR2 appears. This is one of the fundamental

advantages of the LLC resonant converter over the traditional LC series-resonant

converter. In fact it helps keep operation away from capacitive mode. Although the tank

circuit current IR lags the impressed voltage (being R>Rcrit by assumption) so that they

have the same sign at half-bridge leg transitions, the switching period is longer than the

resonant period,1/fR1. Then IR, which is decaying (in absolute value), might come close

to zero or even reverse if it still evolved according to the same sinusoid at frequency

f=fR1 (see the extrapolated black lines drawn in (t1, t2) and (t5, t6)). This lower frequency

sinusoid "holds up" the tank current, hence ensuring that the switched currents IR(t2)

and IR(t6) do not change sign and have amplitude large enough to complete the HB

node swing well within the deadtimes (t2, t3) and (t6, t7) respectively, i.e. ZVS.

In the series LC tank Ls-Cr operating below resonance with R>Rcrit the voltage drop

across the L-C series is negative (the capacitive reactance is larger in module than the

inductive reactance), i.e. a minus sign is located at the node HB. As a result:

Equation 14

Then, when operating below resonance, for a given input voltage, the LLC resonant

half-bridge will provide an output voltage higher than that available at resonance, and

vice versa, with a given output voltage the LLC resonant half-bridge will operate below

a resonance if the input voltage is lower than 2·Vout. In other words, the conversion

ratio, intended as previously mentioned, is > 1, then the LLC is said to have a "step-up"

characteristic when operating below resonance.

The secondary rectifiers D1 and D2 start conducting when Q2 and Q1 are switched off,

respectively. The initial current is zero and its di/dt is low, thus they have a soft turn-on.

out

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